General Antonyms:
* Sad
* Unhappy
* Depressed
* Miserable
* Gloomy
* Melancholy
* Downcast
* Despondent
* Disgruntled
* Wretched
More Specific Antonyms:
* Angry (if happiness is replaced by anger)
* Fearful (if happiness is replaced by fear)
* Anxious (if happiness is replaced by worry)
* Bored (if happiness is replaced by lack of excitement)
* Frustrated (if happiness is replaced by a sense of being blocked)
The best antonym will depend on the specific meaning of "happy" in the context you are using it.